The destruction caused by earthquakes mainly arises from the horizontal components of the oscillation of the earth. These components impose very large shear forces on constructions. When those forces exceed the strength of one structural member, this member is damaged.
In my previous patent application, I described a system for isolating a construction from the earth when horizontal seismic forces exceed a predetermined value. The system includes a base, usually of reinforced concrete, supports which carry the weight or load of the construction and impose elastic forces on the construction which react against horizontal movement, and connecting means which prevent horizontal movement of the construction under normal circumstances, for example, when the building is subjected to forces from winds.
In the specific embodiment described in my previous application, the connecting means is comprised of elongated bars, which are attached to the base and to the ground. The bars are primarily of large diameter but have a short segment of small diameter. The bars are attached in such a way that they are in tension at all times. If the structure is subjected to great horizontal forces, the tensile strength of the short segment is exceeded and it breaks. This releases the construction so that it may oscillate freely, under the influence of the elastic forces described above, somewhat like a giant pendulum.
The means for attaching the bars described in my previous application is comprised of a post which extends upwardly through an opening in the base and anchorages which attach the bars to the base and to the post. Once the bars break, the base can move relative to the post, within the limitations imposed by the size of the opening in the base through which the post projects.
In certain locations, the anticipated oscillations of the base are very great and therefore the opening must be quite large. However, in certain constructions, such large openings cannot be accepted. For example, in a concrete bridge, a large opening in the deck might be needed which would not be acceptable for reasons of economy, because vehicles travel over the deck and therefore the bridging of the openings is very expensive. Also it would be necessary to overcome great practical design difficulties even if the bars are arranged under the deck.